Aircraft power distribution systems are safety critical. Hence they need to be protected from over-current conditions that may be caused, for example, by lightning strikes, short circuits, etc., that might otherwise produce high currents that permanently damage or break wiring or other electrical components.
Over-current protection is often provided by using various devices and systems that trip and break a circuit if a detected current (Ifault) is larger than a maximum operating or rated current (Inormal) in, for example, an aircraft wiring harness.
Low power electrical power distribution (<4 kW) in aircraft is currently performed by SSPCs (Solid State Power Controllers). SSPCs provide a semiconductor switch, which has good performance and characteristics, including: very fast response, limiting of the fault current to within safe limits; a long multi-operation life span; a flexible construction and control scheme, that is fully controllable for both functions of current limiting and interruption; and low-cost with a minimal maintenance requirement.
However to increase the power switching capability of SSPCs a much larger and heavier solution is required (for reasons discussed briefly below), which is why to date the alternative technology of choice for high power aircraft electrical distribution has been the electro-mechanical contactor.
Conventional technology for SSPCs has been based on metal-oxide-silicon field effect transistors (MOSFETs) because of their low on-state resistance which provides low power dissipation (as heat) during normal operation. However, in aircraft systems these devices must be able to survive in harsh transient conditions, which requires the use of high speed control electronics to allow survivable operation.
Additionally, whilst such MOSFET-based systems have various advantages, they also operate at a relatively high temperature (e.g. the MOSFET junction temperature may typically be ˜100° C.). Because of this, there is only a relatively small temperature range (i.e. a window of about 50-60° C.) in which the MOSFET devices need to be turned off in the event of a fault in order to prevent them heating into a temperature range above which silicon stops behaving as a semiconductor (i.e. above about 165° C.).
This narrow temperature switch off range places design limitations on the MOSFET-based system. For example, where Ifault≈10.Inormal, a power surge can increase the heat generated in the MOSFET devices by a factor of about 100 since heat generated is proportional to Pfault (the power to be dissipated Pfault=I2fault.R, where R is the resistance of the MOSFET devices). This heat needs to be effectively dissipated if the MOSFET devices are to remain at a temperature of below about 165° C. so as to be able to function as required.
Therefore conventional systems using MOSFETs are often provided with passive and/or active cooling. For example, forced fluid cooling may be used, as may the provision of one or more heatsink devices. The system may also be formed using many individual MOSFET devices provided in parallel, and/or by using very bulky power devices, in order to provide a large physical amount of semiconductor material that can dissipate any heat generated under fault conditions without provoking a substantial temperature rise in the MOSFETs (essentially by reducing the value of R).
Hence in order to be able to cope with possible over-current conditions, particularly at high power, conventional MOSFET-based power protection systems are generally fairly bulky and heavy. Clearly, this is disadvantageous, particularly in aircraft.